One of the most difficult problems when working with ozone or hydrogen peroxide is the measurement of its exact concentration in a moving gas. The problem becomes more severe if either or both the pressure and temperature of the gas are changing. There are a number of instruments commercially available that will measure ozone but they are complicated, expensive, difficult to calibrate and have a long time constant. These commercial instruments are difficult to design into a system and generally can not be built to be portable.
It should be pointed out that U.S. Pat. No. 3,153,577 of 1964 teaches the use of a DC bridge circuit for measuring ozone concentration when the detector is a thermistor. This patent does not measure or control flow, temperature or pressure.
U.S. Pat. No. 4,409,183 of 1983 covers a method to measure ozone in water. However, different dissolvable salts will produce errors in the measurements.
The U.S. Pat. No. 2,899,281 teaches the use of a catalyst coated thermistor employed in a bridge, as does U.S. Pat. No. 3,153,577. Neither of these patents takes the pressure of their samples into account in their measurements and calculations, nor does either patent employ a heat sink with or at their thermistors. By not taking both temperature and pressure into the concentration calculations, they can not measure an ozone concentration of 2 parts per million with any accuracy or confidence.
The U.S. Pat. No. 3,464,797 of 1966 teaches the measuring of ozone by a charcoal catalyst held in a tube. This is fine, but this patent has not taken into account either the gas temperature or its pressure and it does not teach how to control its gas or how it controls or measures the time constant. This system's sensitivity is also low.
The common problem with most of the commercial ozone monitoring instruments available is that they do not cover a broad enough range and are to slow to respond. For example, in the wood pulp industry, the need is for an instrument with a fast time constant that will measure ozone concentration and also be the detector to control the total bleaching system. In this case, the ozone is (in its gas phase operating at a temperature from 5 to 25 degrees centigrade at a concentration up to 20%) employed as a bleaching agent on wood pulp. The ozone is administered quickly so as not to destroy the fibers. It is therefore necessary to quickly know the ozone concentration at different temperatures and pressures. The problem is measuring and following any change in concentration at high concentrations at different pressures and at changing temperatures. These measurements must be done continuously, as part of the system, with an instrument having a short time constant of less than 0.25 seconds.
Another point, is that most of the available instruments for monitoring ozone or hydrogen peroxide are not meant to be portable. The need, when working with ozone or hydrogen peroxide, is for a portable instrument that can be used to sniff out where an O.sub.3 or an H.sub.2 O.sub.2 leak may occur. This portable instrument should be sensitive, (less than 1 part per million full scale), have a short time constant with an accuracy of plus or minus 2% or less, and be battery powered. It should also be equipped with a long intake tube that can be used to reach out to check for leaks around flanges and valves, for example, in a city's drinking water purification systems employing ozone as a sterilant or in a waste water disinfection plant. In all cases, both monitors, the stationary installed ozone system monitor or the hand held portable model, should be completely automatic, need little or no calibration, be simple and need only minor servicing.